The distribution of silver halide grain sizes within a radiation sensitive silver halide emulsion is recognized as a fundamental determinant of its properties. This can be illustrated by reference to FIG. 1 wherein a characteristic curve described by James and Higgins, Fundamentals of Photographic Theory, Wiley, 1948, p. 180, is shown. Within the segment BC of the characteristic curve density increases linearly with tne logarithm of exposure. The exposure range MN constitutes the exposure latitude of the emulsion. As exposure is decreased below level M reductions in density become progressively less until point A on the characteristic curve is reached below which no further decrease in density is observed. Thus, the density at point A corresponds to the minimum density, D.sub.min, of the emulsion. The segment AB is referred to as the toe of the characteristic curve. If exposure is increased beyond N, increases in density become progessively less until a point D is reached beyond which no further increase in density is observed. Thus, the density at point D corresponds to the maximum density, D.sub.max, of the emulsion. The segment CD is referred to as the shoulder of the characteristic curve. The tangent of the angle .alpha., referred to as .gamma., is a way of describing the slope of the characteristic curve.
If all of the silver halide grains present in the emulsion were exactly the same size and identically sensitized, the segment BC of the characteristic curve would approach the vertical--i.e., .gamma. would be extremely high. Exposure latitude MN would be extremely narrow. Broader exposure latitude is observed in actual emulsions largely because a distribution of silver halide grain sizes are present in silver halide emulsions. The density increase in the toe and adjacent portion of tne characteristic curve results from the disproportionate response of larger silver halide grains to lower levels of exposure while the density increase in the shoulder and adjacent portion of the curve is the result of the smaller silver halide grains reaching their latent image forming threshold on exposure.
An idealized response for a silver halide emulsion would be a characteristic curve that is linear in both its toe and shoulder, as indicated by A'B and CD', thereby extending its exposure latitude. One explanation for the density of A lying above A'--i.e., elevated minimum density levels--is that the tendency toward spontaneous development of silver halide grains increases as the size of the grains increases. Similarly, an explanation for the density disparity between D and D' is the presence of grains too small to contribute usefully to photographic imaging.
From the foregoing it is apparent that a controlled distribution of silver halide grains is desirable to select exposure latitude. At the same time it is apparent that both the very largest and the very smallest grains present in an actual silver halide emulsion contribute only marginally to imaging. While FIG. 1 depicts the characteristic curve of a negative working silver halide emulsion, essentially similar relationships can be identified and conclusions drawn from the characteristic curve of a direct positive silver halide emulsion.
Although fundamentally important to controlling imaging, the distributions of silver halide grain sizes in the emulsions of photographic elements have represented accommodations to manufacturing capabilities rather than grain size distributions that would have been chosen given an unrestrained freedom of choice. The art has long employed for differing photographic applications silver halide emulsions ranging in mean diameter over approximately three orders of magnitude--e.g., 0.03 .mu.m for high resolution film to about 2.5 .mu.m for medical X-ray film. Recently developed high aspect ratio tabular grain emulsions have extended useful grain diameters upwardly by at least another order of magnitude. For some applications, such as lithographic films, high gammas (typically greater than 10) and high image discrimination (Dmax-Dmin) are required while for other applications, such as camera films and medical X-ray films, much lower gammas (typically 1.5) and extended exposure latitudes (2 log E or greater) are sought. However, in each of these emulsions the silver halide grain distribution is constituted by a peak frequency of grains at or near the mean diameter with numerous additional grains being present departing from the peak frequency size by an error distribution, typically a Gaussian (i.e., normal) distribution.
Characteristically the formation of a silver halide grain population in manufacturing a photographic emulsion is the result of silver halide precipitation, wherein silver and halide ions react to form silver halide, and physical ripening, wherein the grains attain approximately their final size and form. While ripening can and does occur to some extent concurrently with precipitation, it is in general a slower step that requires holding the emulsion for a period of time following the termination of precipitation.
Single jet precipitation procedures are recognized to produce silver halide grains of an extended range of sizes. FIG. 2 is an illustration of a neutral octahedral silver bromoiodide emulsion and FIG. 3 is an illustration of an ammoniacal cubic bromoiodide emulsion, each prepared by single jet precipitation. These illustrative emulsions are described by Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66 through 74. Single jet precipitation runs silver salt into a reaction vessel containing the halide salt. While this produces a wide distribution of grain sizes, it also inherently results in the excess of halide ions continuously varying throughout the run with attendant non-uniformity in grain crystal structures.
To obtain better control over the silver halide precipitation reaction silver halide emulsions have been increasingly prepared by double jet precipitation techniques. By this technique silver and halide ions are concurrently introduced into a reaction vessel containing a dispersing medium and, usually, a small portion of halide salt used to provide a halide ion excess. Double jet precipitation has the advantage of allowing silver and halide ion concentrations, usually expressed as the negative logarithm of silver or halide ion activity (e.g., pAg or pBr) to be controlled, thereby also controlling the grain crystal structure.
A second important characteristic of double jet precipitation is that it can produce a narrower size distribution of silver halide grains than single jet precipitation. This is an advantage when higher gamma emulsions are sought, but a disadvantage when extended exposure latitudes are desired. Double jet precipitation, though allowing compression of the range of grain sizes present, also produces a normal or Gaussian error distribution of grain sizes.
Silver halide emulsions of narrower and broader grain size distributions are often distinguished by being characterized as "monodisperse" and "polydisperse" emulsions, respectively. Emulsions having a coefficient of variation of less than 20% are herein regarded as monodisperse. Emulsions intended for applications requiring extremely high .gamma. often require coefficients of variation below 10%. As employed herein the coefficient of variation is defined as 100 times the standard deviation of the grain diameters divided by the mean grain diameter. From this definition it is apparent that as between emulsions of identical coefficients of variation those having lower mean grain diameters exhibit a lower range of grain sizes present. For this reason the error distribution of grain sizes in monodisperse fine grain emulsions--that is, those less than about 0.2 .mu.m in mean grain diameter--is typically regarded for practical purposes as negligible. However, as mean grain diameter increases not only does absolute divergence in grain sizes increase at a given coefficient of variation, but also it becomes increasingly difficult to obtain low coefficients of variation. It is, for example, relatively more difficult to achieve low coefficients of variation in preparing high aspect ratio tabular grain emulsions.
Although double jet precipitation is normally practiced as a batch process, it is possible to withdraw product emulsion continuously while concurrently introducing reactants, thereby transforming the process into a continuous one. In this latter instance the size-frequency distribution curve becomes asymmetrically distorted, as shown by the illustrative curve in FIG. 4. (Plotting diameter on a logarithmic scale can be undertaken to obtain a more symmetrical curve.) However, like the product emulsion of each of the preceding precipitation processes, the size-frequency distribution curve of the product emulsion exhibits an error distribution of grain sizes that is dictated by the precipitation process employed.
Because of the limitations of silver halide grain formation processes, post formation adjustments are commonly employed to improve product emulsion grain size distributions and thereby achieve aim characteristic curves. For example, increasing the proportion of relatively larger or smaller silver halide grains in an emulsion fraction can be achieved by hydrocyclone separation techniques. More commonly, particularly in extending exposure latitude, separately prepared and sensitized emulsions are blended (or coated in separate layers) to obtain an aim characteristic curve. Trial and error sensitization and blending or coating are required to achieve the aim characteristic curve shape. Post formation adjustments of silver halide grain distributions add significantly to the complexity of preparing useful radiation sensitive emulsions and photographic elements. Even so, process of precipitation imposed limitations on silver halide grain size distributions are merely modified, not eliminated, by post formation adjustments.
Considering the fundamental importance of silver halide grain size distribution and the limited success achieved in the art in modifying grain size distributions, it is not surprising that a plethora of variant silver halide precipitation schemes have been advanced over the years. The following, primarily directed to variants of double jet precipitation techniques, are considered illustrative of the prior state of the art:
P-1 Frame et al U.S. Pat. No. 3,415,650 discloses a basic double jet precipitation apparatus with an efficient stirring device.
P-2 Miyata U.S. Pat. No. 3,482,982 discloses the addition of iodide ions either in crystalline or soluble salt form during single jet precipitation of silver bromoiodide.
P-3 Irie et al U.S. Pat. No. 3,650,757 discloses the double jet precipitation of monodisperse silver halide emulsions with accelerated rates of silver and halide salt introductions.
P-4 Posse et al U.S. Pat. No. 3,790,386 and Forster et al U.S. Pat. No. 3,897,935 disclose the double jet precipitation of silver halide emulsions while circulating between grain nucleation and growth zones.
P-5 Terwilliger et al U.S. Pat. No. 4,046,576 discloses a continuous double jet precipitation process.
P-6 Maternaghan U.S. Pat. No. 4,184,878 discloses employing preformed high iodide silver halide grains in preparing tabular grain emulsions.
P-7 Saito U.S. Pat. No. 4,242,445 discloses increasing the concentrations of soluble silver, halide, or silver and halide salts during double jet precipitation of monodisperse silver halide emulsions.
P-8 Mignot U.S. Pat. No. 4,334,012 and Brown et al U.S. Pat. No. 4,336,328 disclose performing ultrafiltration during the course of double jet precipitation, either in a unitary reaction vessel arrangement or in an arrangement employing grain nucleation and growth zones.
P-9 Japanese application No. 65799/66, filed Oct. 6, 1966, discloses preparing a highly sensitive, high .gamma. emulsion by adding a silver chloride emulsion as well as silver and halide salts to prepare a negative working emulsion.
P-10 U.K. Pat. No. 1,170,648 discloses preparing a silver halide emulsion by placing silver halide seed grains in the reaction vessel before running in silver and halide salts.
The preparation of silver halide emulsions intended to trap photogenerated electrons within the interior of the grains, most frequently employed for direct positive imaging, is generally recognized to be more complex than preparing negative working silver halide emulsions in which the photogenerated electrons form surface latent images predominantly on the surfaces of the grains. This is particularly true when moderate or longer exposure latitudes are required. Commonly employed direct positive emulsions which rely on internal trapping of electrons are those (a) in which the surfaces of the grains are fogged and photogenerated holes are relied upon to bleach surface fog and (b) in which internally trapped electrons form a desensitizing internal latent image that retards surface development. The higher speed direct positive emulsions are of the latter type and rely on silver halide grains which are surface sensitized, but in a controlled manner that preserves the internal latent image forming characteristic of the grains. This is often achieved by forming a monodisperse core emulsion which is either doped or surface sensitized, shelling this core emulsion with additional silver halide, and surface sensitizing to a limited extent the final core-shell grains to increase their sensitivity. When an aim characteristic curve requires the preparation and blending of a plurality of direct positive emulsions, particularly core-shell emulsions, it can be readily appreciated that emulsion preparation can become exceedingly laborious. The following are illustrative of the prior state of the art:
P-11 Berriman U.S. Pat. No. 3,367,778 discloses a direct positive core-shell silver halide emulsion the grains of which are surface fogged rather than being surface sensitized.
P-12 Evans U.S. Pat. No. 3,761,276 discloses a direct positive core-shell silver halide emulsion the grains of which are surface sensitized.
P-13 Atwell et al U.S. Pat. No. 4,269.927 discloses a direct positive core-shell silver halide emulsion prepared by blending emulsions of differing core sensitization.